The present invention relates to a high frequency magnetic field generator for nuclear magnetic resonance (NMR) or electron spin resonance (EPR) and, particularly, to a high frequency coil device having transmission coil means, receiving coil means or transmission/receiving coil means.
FIG. 1 shows schematically a conventional high frequency magnetic field generator for NMR. The magnetic field generator includes a high frequency amplifier 2 connected to a high frequency input terminal, an impedance matching circuit 3 connected to the high frequency amplifier 2 and a high frequency magnetic field generating portion composed of a pair of opposing and parallel connected high frequency coils 1 having opposite ends connected to the impedance matching circuit 3. Each coil takes in the form of saddle making an angle of 120.degree., generally.
A high frequency signal supplied to the high frequency input is amplified by the high frequency amplifier 2. An output of the amplifier 2 is supplied through the impedance matching circuit 3 to the high frequency coils 1 by which a vibrating magnetic field is produced in a region defined therebetween, which is time-dependent similarly to high frequency current supplied thereto. The impedance matching circuit 3 serves to match an output impedance of the amplifier 2 to an impedance of the coils 1 to thereby provide a smooth transmission of the amplified signal between them.
In a receiving operation mode, a NMR signal emitted from the substance disposed in the region defined between the coils 1 is received by these coils. The NMR signal produces a rotating magnetic field by which a high frequency sinusoidal current is induced in the coils. The induced current is tuned by the matching circuit 3 and sent through a preamplifier (not shown) to a suitable receiving device by synchronous electrical switches.
In the conventional high frequency magnetic field generator for NMR, high frequency magnetic field to be generated is a linearly polarized vibrating field and, assuming that an amplitude of rotating magnetic field is .sqroot.2 B, an amplitude of vibrating magnetic field is 2 .sqroot.2 B. Therefore, only one of two opposite rotating magnetic fields of the vibrating magnetic field can be utilized as an effective rotating magnetic field for NMR or EPR. That is, only half of the power supplied can be used as an effective power. Further, a receiving efficiency of NMR signal from the substance is relatively low and an S/N ratio is determined by a noise level of the coils 1, so that noise is supplied directly to the receiving device.